The present invention is directed to dynamic fixation assemblies for use in bone surgery, particularly spinal surgery, and in particular to longitudinal connecting members and cooperating bone anchors or fasteners for such assemblies, the connecting members being attached to at least two bone anchors.
Historically, it has been common to fuse adjacent vertebrae that are placed in fixed relation by the installation therealong of bone screws or other bone anchors and cooperating longitudinal connecting members or other elongate members. Fusion results in the permanent immobilization of one or more of the intervertebral joints. Because the anchoring of bone screws, hooks and other types of anchors directly to a vertebra can result in significant forces being placed on the vertebra, and such forces may ultimately result in the loosening of the bone screw or other anchor from the vertebra, fusion allows for the growth and development of a bone counterpart to the longitudinal connecting member that can maintain the spine in the desired position even if the implants ultimately fail or are removed. Because fusion has been a desired component of spinal stabilization procedures, longitudinal connecting members have been designed that are of a material, size and shape to largely resist bending (including flexion and extension), torsion, distraction and compression, and thus substantially immobilize the portion of the spine that is to be fused. Thus, longitudinal connecting members are typically uniform along an entire length thereof, and usually made from a single or integral piece of material having a uniform diameter or width of a size to provide substantially rigid, inelastic support in all planes.
An alternative to fusion and the use of hard, rigid longitudinal connecting members or other stiff inelastic structures which immobilize at least a portion of the spine, has been a “soft” or “dynamic” stabilization approach in which a bendable and often resilient loop-, S-, C- or U-shaped member or a coil-like and/or a spring-like member is utilized as an elastic longitudinal connecting member fixed between a pair of pedicle screws in an attempt to create, as much as possible, a normal loading pattern between the vertebrae in flexion, extension, distraction, compression, side bending and torsion. Another type of soft or dynamic system known in the art includes bone anchors connected by cords or strands, typically made from a plastic material. Such a cord or strand may be threaded through cannulated spacers that are disposed between adjacent bone anchors when such a cord or strand is implanted, tensioned and attached directly to the bone anchors. The spacers typically span the distance between bone anchors and cooperate with the tensioned cord to provide limited protected movement, thereby strengthening and supporting a region of the spine. Such cord or strand-type systems require specialized bone anchors and tooling for tensioning and holding the cord or strand in the bone anchors. Although bendable, the cords or strands and spacers utilized in such systems do not allow for any significant elastic compression and distraction or elongation of the system in a caudad and/or cephalad direction once implanted because the spacer must be significantly precompressed and the cord or strand must be substantially stretched or pulled to maximum tension in order to provide a stable, supportive system. Such tensioned cord and precompressed spacer systems may also cause undesirable facet joint compression loading during certain spinal movements, especially side bending and flexion, due primarily to this lack of posterior spinal column elongation which significantly alters the normal biomechanics of segmental spinal motion.
The complex dynamic conditions associated with spinal movement create challenges for the design of elongate compressible and/or distractable longitudinal connecting members that exhibit an adequate fatigue strength to provide internal fixation and stabilization for protected motion of the spine, without fusion, and that allow for some natural movement or at least some modified protected motion of the portion of the spine being reinforced and supported by such an elongate connecting member that can be partially or fully elastic. A further challenge are situations in which a portion or length of the spine requires a more rigid stabilization, possibly including fusion, while another portion or length may be better supported by a less stiff and more dynamic system that allows for modified protected spinal motion, again, without fusion.